This invention relates to voltage-controlled resistors and more particularly to unmatched field effect transistors (FET's) providing matched voltage-controlled resistances.
The use of a single FET as a voltage-controlled resistor is described in the article, "FET's As Voltage-Variable Resistors" by Carl D. Todd, Electronic Design, Sept. 13, 1965, pp. 66-69. In such applications, the FET drain-to-source voltage is restricted to be less than the pinch-off voltage such that it operates in the so-called triode region below the knee of the performance characteristics of the FET (I.sub.D vs. V.sub.DS, with V.sub.GS constant). The pinch-off voltage V.sub.GSO may be defined as the gate-to-source voltage that reduces the drain current to 0.01 of its zero bias value, the drain being maintained at a positive potential with respect to the source which is greater than the magnitude of the pinch-off voltage. The FET performance characteristics are generally symmetrical through the origin. The relationship between drain current I.sub.D and drain-to-source V.sub.DS is approximately linear over a limited range of this current and voltage, where the channel pinch-off conditions are not too closely approximated and the gate-to-source channel is reverse biased. Thus, the FET operates in this limited region as a voltage-controlled variable resistor to a small AC signal that is applied across the drain-to-source channel thereof.
A cascaded attenuator using a pair of FET's as variable resistors is described in a Siliconix application note, "FET's As Voltage-Controlled Resistors" by Dave Capella and carrying the date February 1973. The circuit including the cascaded attenuator there is purely resistive, i.e., it does not include any reactive elements. This prior-art attenuator circuit is similar to the circuit in FIG. 1, except that the elements in the boxes 3 and 4 are purely resistive and that it does not include the other resistors in FIG. 1. Although this prior-art attenuator provides a variable resistance which is a function of a control voltage, the resistances provided by individual FET's there are not matched and need not be matched for proper operation of the circuit. More specifically, the FET's do not normally each present the same drain-to-source channel resistance for the same control voltage over a range of control voltages. In a circuit such as in FIG. 1 which includes reactive elements in the boxes 3 and 4, it is desirable that the resistances provided by the FET's be matched, i.e., that each FET present the same drain-to-source resistance for a given control voltage. The matching is necessary in this instance to provide a predictable family of circuit responses by applying the appropriate control voltage. Since the characteristics of FET's vary considerably between units, FET's having similar characteristics must be individually selected for use in such applications. Such a selection process is time consuming and expensive. Although pairs of selected FET's that are matched within 5% over a range of resistances are commercially available, such a matched pair of FET's may cost an order of magnitude more than unmatched FET's.
An object of this invention is the provision of matched voltage-controlled resistors with unmatched FET's.